Semiconductor light emitters include light emitting diodes (LEDs) and laser diodes. LEDs have many advantages over incandescent and fluorescent light sources. LEDs have a relatively small size, low power consumption for a given light output, and a longer lifetime. In addition, LEDs have been attractive candidates for applications in which a colored light source is needed, since LEDs emit light in a relatively narrow band of wavelengths. However, the lack of an inexpensive LED to replace white light sources has hampered the movement of LEDs into these applications.
A semiconductor-based light source that emits light that is perceived by a human observer to be “white” can be constructed by providing a source that emits a combination of blue and yellow light in the proper intensity ratio. For example, high intensity blue-emitting LEDs are known to the art. Yellow light can be generated from the blue light by converting some of the blue photons via an appropriate phosphor. In one design, a transparent layer containing dispersed particles of the phosphor covers an LED chip. The phosphor particles are dispersed in a potting material that surrounds the light-emitting surfaces of the blue LED. A portion of the light leaving the LED is converted to yellow by the phosphor particles. The cost of the phosphors used to convert the blue light to yellow light represents a significant fraction of the cost of the device.
In addition, the phosphor utilized for the wavelength conversion ages over the lifetime of the light source. This aging results in a decrease in the efficiency with which the activation light is converted. Hence, the ratio of blue to yellow light changes over the lifetime of the device as the phosphor ages. This leads to an undesirable color shift in the light source.
White light sources based on UV emitting semiconductor devices are also known in the art. In these devices, the light emitter is covered with a layer having two phosphors, one that generates blue light and one that generates yellow light. Once again, to be perceived as a white light source, the phosphors must be present in a specific ratio. Unfortunately, the phosphors used in these prior art devices age at different rates. Hence, these light sources exhibit the same type of undesirable color shift as the one phosphor devices.